1. Field of the Invention
This invention relates to the field of electronically tunable optical filters utilizing acousto-optic effect in a birefringent crystal.
2. Description of Prior Art
Electronically tunable optical filters have been constructed so that a cone of light of first polarization is diffracted by an acoustic wave in a birefringent crystal to shift from the first to a second polarization of the light beam at a selected bandpass of optical frequencies. The center wavelength of the passband of this type of filter is electronically tunable by changing the frequency of the acoustic wave within the crystal.
Two basic types of tunable acousto-optic filters have been constructed: collinear and noncollinear. In the collinear filter, the incident and diffracted light beams inside the birefrigent crystal are collinear with the acoustic beam. The diffracted light beam at the selected passband is separated from the incident light beam by crossed polarizers. The collinear type of acousto-optic filter is disclosed in an article entitled "Acousto-Optic Tunable Filters" appearing on pages 744-747 in the June, 1969 issue of The Journal of the Optical Society of America (Vol. 59, No. 6), and in U.S. Pat. No. 3,679,288 entitled "Tunable Acousto-Optic Method and Apparatus."
In the noncollinear filter, the light beams inside the birefringent crystal are noncollinear with the acoustic beam. The diffracted light beam at the passband is selected from the incident light beam by either crossed polarizers or spatial separations. The noncollinear type of acousto-optic filter is disclosed in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture" appearing on pages 370-372 of the Oct. 15, 1974 issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121 entitled "Noncollinear Tunable Acousto-Optic Filter."
In both the colliner and noncollinear types of tunable acousto-optic filters, the acousto-optic diffraction occurs for a narrow-band of optical frequencies satisfying the relation that the sum of the momentum vectors of the incident light waves and acoustic waves equal the momentum vector of the diffracted light wave. More significantly, this narrow filter bandpass can be maintained for incident light having a distribution of incident directions. This large angular aperture characteristic is due to the proper choice of acousto-optic interaction geometry wherein the tangents to the loci of incident and diffracted light wavevectors are parallel. When the "parallel tangents" condition is met, the acousto-optic diffraction becomes relatively insensitive to the angle-of-light incidence, a process that is referred to as "noncritical phase-matching." For the remainder of this disclosure, "tunable acousto-optic filter" will be defined as an optical filter that operates on the basis of acousto-optic diffraction in a birefringent crystal wherein the non-critical phase matching condition is satisfied. This type of tunable acousto-optic filter is clearly distinguishable from the type with small angular aperture as described in an article entitled "New Noncollinear Acousto-Optic Tunable Filter Using Birefringence in Paratellurite" appearing on pages 256-257 of the Mar. 15, 1974 issue of Applied Physics Letters (Vol. 24), and in U.S. Pat. No. 3,953,107 entitled "Acousto-Optic Filters." The latter device is basically a narrowband deflector rather than a filter since its use must be restricted to well collimated light sources.
The tunable acousto-optic filters constructed so far have used birefringent crystals that include LiNbO.sub.3, CaMoO.sub.4, crystal quartz, TeO.sub.2 and Tl.sub.3 AsSe.sub.3. Tunable acousto-optic filters using these filter materials have been found to exhibit a trade-off relation between the spectral and angular bandwidths. For a tunable acousto-optic filter with a bandwidth of 10 .ANG. in the visible spectrum region, the corresponding angular aperture is typically a few degrees.
For many applications, such as laser communications, it is desirable that the tunable acousto-optic filter can provide a very narrow bandwidth (.about.1 .ANG.) and an extremely large angular aperture (.about.45 degrees). This requirement cannot be met with the prior art of tunable acousto-optic filters. Accordingly, it is fair and accurate to state that what it is needed in the art and is not available is a tunable acousto-optic filter with enhanced spectrum resolution and increased angular aperture.